This invention relates to the field of processing of fluoroelastomer compositions.
In a never-ending search for improved materials, attention has been focused on fluoroelastomers because of their inherent chemical inertness. Of course, fluoroelastomers in general are well known. In this regard, see "The Vanderbilt Rubber Handbook," Robert O. Babbit, R. T. Vanderbilt Co., Inc., pp. 244-253 (1978). Unfortunately, this desirable inherent chemical inertness causes problems of its own. One problem is that blending anything into them is unusually difficult. In certain fluoroelastomers, for example, it is difficult to blend more than 1 or 2% of a processing oil, wax, or lubricant into the material without it bleeding to the surface in a relatively short time.
Another problem relates to their processing, in particular blow molding and extrusion. Blow molding and extrusion are, of course, manufacturing processes well known to those skilled in the art. Certain fluoroelastomers cannot be blow molded because they do not knit well at weld lines and can only be extruded with difficulty due to their high molecular weight. In general, rubbers are not blow molded whereas plastics are. Some thermoplastic elastomers can be blow molded but these are considered to be hybrids between rubbers and plastics.
A further problem with certain fluoroelastomers is their poor flexibility (i.e., loss of elasticity) at low temperatures. This poor low temperature flexibility excludes these fluoroelastomers from use by the military and oil exploration groups where exposure to extreme cold environments is expected.
Two specific fluoroelastomers of interest are VITON.RTM. (a product of E. I. du Pont de Nemours and Co., Wilmington, Del.), and AFLAS.RTM. (a product of Asahi Glass Co., Ltd., Tokyo, Japan), both of which are known. See, for example, the above "Vanderbilt Rubber Handbook." In general, VITON is a copolymer of vinylidene fluoride (CH.sub.2 CF.sub.2) and hexafluoropropylene (C.sub.3 F.sub.6) and AFLAS is a copolymer of tetrafluoroethylene (C.sub.2 F.sub.4) and propylene (C.sub.3 H.sub.6).
Both VITON and AFLAS suffer all of the above problems, to wit, difficulty in processing and loss of elasticity at low temperatures. For example, neither material can be successfully blow molded due to poor knitting at the weld lines. Too, VITON becomes functionally brittle at about -40.degree. C. and AFLAS at about -5.degree. C.
Attempts have been made to solve the above problems with additives and also by blending with other polymers such as fluorosilicones. Use of additives is discussed, in general, in "Modern Plastics Encyclopedia," Volume 58, Number 10A, pp. 202-224 (October 1981). At least with respect to VITON and AFLAS, these attempts have largely been unsuccessful.
Thus, it is an object of this invention to have a method of processing a fluoroelastomer composition so that the composition is flexible at low temperatures.
It is another object of this invention to have a method of processing a fluoroelastomer composition wherein the processing is by blow molding or by extruding.
These and other objects of the invention will become apparent after reference to the following description.